Definition
A Locator Outer Marker (LOM) is a navigation facility that combines a low-power non-directional beacon (NDB) with an outer marker beacon, located at the outer marker position of an Instrument Landing System (ILS) approach, typically 4 to 7 miles from the runway threshold. The NDB portion provides bearing guidance to help the aircraft intercept the ILS final approach course, while the outer marker portion transmits a 75 MHz signal straight up that triggers a cockpit indication when the aircraft passes overhead.
Plain English
A ground station near the start of an ILS approach that does two jobs: it acts as a homing beacon to help guide you onto the final approach line, and it tells you when you have flown directly over it.
Context Anchor
Seen on instrument approach charts and in instrument procedure naming, especially on older procedures that use marker beacons or locator beacons.
Derivation
Locator' refers to the NDB's role in helping the pilot locate and intercept the final approach course. 'Outer marker' describes its position — the outermost of the marker beacons along the approach path. Together the name tells you exactly what the facility does and where it sits.
Why Pilots Care
They allow pilots to confirm their location and begin the final descent on precision approaches in low visibility.
Intuition Check
Do not read “outer marker” as just any marker outside the airport. Here it means a specific radio marker point on an instrument approach.
Example Sentence 1
Cleared for the ILS approach, the pilot tuned the ADF to the LOM frequency and began tracking inbound to intercept the localizer.
Example Sentence 2
Approaching in clouds, the crew identified the LOM signal to know their distance from the runway.